1. Halloween party triggers at 550 corruption. Ash is calling Kelly. Kelly needs to buy the witchcostume.
Party starts saturday 3. daytime. Logic is broken here. You need to click work in cafe or enter home for example first. After this you can click the dorm on city map to start the party. If you passed the 3. daytime by other events or because of missing witchcostume you need to wait for next saturday.